The present invention relates to electrochemical cells, and more particularly to flow field membrane for supporting membrane components and enhancing fluid flow in electrochemical cells.
Electrochemical cells are energy conversion devices, usually classified as either electrolysis cells or fuel cells. A proton exchange membrane electrolysis cell can function as a hydrogen generator by electrolytically decomposing water to produce hydrogen and oxygen gas, and can function as a fuel cell by electrochemically reacting hydrogen with oxygen to generate electricity. Referring to FIG. 1, which is a partial section of a typical anode feed electrolysis cell 100 (“cell 100”), process water 102 is fed into cell 100 on the side of an oxygen electrode (anode) 106 to form oxygen gas 104, electrons, and hydrogen ions (protons) 106. The reaction is facilitated by the positive terminal of a power source 120 electrically connected to anode 116 and the negative terminal of power source 120 connected to a hydrogen electrode (cathode) 114. The oxygen gas 104 and a first portion 108 of the process water exit cell 100, while protons 106 and a second portion 110 of process water migrate across a proton exchange membrane 118 to cathode 114 where hydrogen gas 112 is formed.
Another typical water electrolysis cell using the same configuration as is shown in FIG. 1 is a cathode feed cell, wherein process water is fed on the side of the hydrogen electrode. A portion of the water migrates from the cathode across the membrane to the anode where hydrogen ions and oxygen gas are formed due to the reaction facilitated by connection with a power source across the anode and cathode. A portion of the process water exits the cell at the cathode side without passing through the membrane.
A typical fuel cell uses the same general configuration as is shown in FIG. 1. Hydrogen, from hydrogen gas, methanol, or other hydrogen source, is introduced to the hydrogen electrode (the anode in fuel cells), while oxygen, or an oxygen-containing gas such as air, is introduced to the oxygen electrode (the cathode in fuel cells). Water can also be introduced with the feed gas. Hydrogen gas electrochemically reacts at the anode to produce protons and electrons, wherein the electrons flow from the anode through an electrically connected external load, and the protons migrate through the membrane to the cathode. At the cathode, the protons and electrons react with oxygen to form water, which additionally includes any feed water that is dragged through the membrane to the cathode. The electrical potential across the anode and the cathode can be exploited to power an external load.
In other embodiments, one or more electrochemical cells can be used within a system to both electrolyze water to produce hydrogen and oxygen, and to produce electricity by converting hydrogen and oxygen back into water as needed. Such systems are commonly referred to as regenerative fuel cell systems.
Electrochemical cell systems typically include one or more individual cells arranged in a stack, with the working fluids directed through the cells via input and output conduits formed within the stack structure. The cells within the stack are sequentially arranged, each including a cathode, a proton exchange membrane, and an anode (hereinafter “membrane electrode assembly”, or “MEA”). Each cell typically further comprises a first flow field in fluid communication with the cathode and a second flow field in fluid communication with the anode. The MEA may be supported on either or both sides by porous screen packs or bipolar plates disposed within the flow fields, and which may be configured to facilitate membrane hydration and/or fluid movement to and from the MEA. In addition, to maintain intimate contact between cell components especially across the MEA, uniform compression is applied to the cell components using pressure pads or other compression means to provide even compressive force from within the electrochemical cell.
In addition to providing mechanical support for the MEA, flow field members such as screen packs and bipolar plates preferably facilitate fluid movement and membrane hydration. In a fuel cell for example, process water is directed toward the anode, which must remain wet for optimal performance. Process water transported to the cathode via attachment to hydrogen ions is preferably directed away from the cathode, as well as any resultant water. Porous flow field members can also serve as gas diffusion media to effectuate proper transport of the oxygen and hydrogen gas. Increasing the rates of transport and uniformity of distribution of the cell fluids (e.g. liquid water, oxygen gas and hydrogen gas) throughout the electrochemical cell increases operating efficiencies.
Conventionally, porous flow field members have been formed of carbon-based materials such as graphite. However, the carbon-based materials are subject to oxidative degradation due to the presence of oxygen, thereby resulting in decreased cell performance. Porous titanium supports have also been used, as disclosed in publications of the General Electric Company, specifically “Solid Polymer Electrolyte Water Electrolysis Technology Development for Large-Scale Hydrogen Production,” Final Report for the Period October 1977–November 1981 by General Electric Company, NTIS Order Number DE82010876, e.g., at pages 66 and 90. Porous titanium supports were shown to improve water flow rates and current densities compared to cells employing perforated foils. Additionally, “Industrial and Government Applications of SPE Fuel Cell and Electrolyzers” by T. G. Cooker, A. B. LaConti and L. J. Nuttall (General Electric Company) presented at the Case Western Symposium on “Membranes and Ionic and Electronic Conducting Polymer,” Cleveland, Ohio May 17–19, 1982, e.g., page 14, discloses use of a porous, rigid titanium sheet on the anode and carbon fiber paper on the cathode for the purpose of preventing the membrane and electrode assembly from deforming into the flow fields.
While existing porous flow field members are suitable for their intended purposes, there nonetheless remains a perceived need for improved porous flow field members, in particular more robust members with improved water and gas transport properties.